In the field effect transistor, electric conductivity of a semiconductor layer which is provided between two electrodes, a source electrode and a drain electrode, is controlled by voltage that is applied to a gate electrode. The field effect transistor is basically a typical unipolar element in which a p-type or n-type carrier (a hole or an electron) transports electric charge.
Since various switching elements or amplifier elements can be formed by combination of such field effect transistors, the field effect transistor is applied in various fields. For example, a switching element of a pixel in an active matrix display or the like can be given as an application example.
So far, as a semiconductor material used for the field effect transistor, an inorganic semiconductor material typified by silicon has been widely used; however, high temperature processing is necessary to form an inorganic semiconductor material as a semiconductor layer. Therefore, it is difficult to use plastics or a film for a substrate.
On the other hand, when an organic semiconductor material is used as a semiconductor layer, the material can be formed at relatively low temperature. Therefore, it becomes theoretically possible to manufacture a field effect transistor over not only a glass substrate but also a substrate having low heat resistance, such as a plastic substrate.
As an example of a field effect transistor using an organic semiconductor material as a semiconductor layer (hereinafter referred to as an organic field effect transistor), a transistor which uses silicon dioxide (SiO2) as a gate insulating layer and pentacene as a semiconductor layer is given (see Non Patent Document 1: Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Letters, Vol. 18, 606-608 (1997)). In this repot, it has been reported that field effect mobility is 1 cm2/Vs, and transistor performance that is equal to that of a transistor using amorphous silicon can be obtained even when an organic semiconductor material is used as a semiconductor layer.
In the organic field effect transistor, carriers are transported between a source and drain electrode and a semiconductor layer. When there is an energy barrier at the interface, transistor characteristics such as field effect mobility are lowered. In order to improve this situation, it has been proposed to use a lithium fluoride layer at the interface between the source electrode or the drain electrode and the semiconductor layer (see Patent Document 1: Japanese Published Patent Application No. 2003-298056). However, the lithium fluoride layer can be only applied to an n-channel organic field effect transistors and the type of the organic semiconductor material is limited to n-type materials. In addition, it has also been proposed to dope a semiconductor layer with a conductivity imparting agent (see Patent Document 2: Japanese Published Patent Application No. 2004-228371); however, there is a problem in that a conductivity imparting agent has low chemical stability. Moreover, adhesion between these electrode materials and organic semiconductor materials is important in order to obtain a transistor having excellent durability.
As described above, a source electrode and a drain electrode which can be used for an organic field effect transistor using various organic semiconductor materials, which are chemically stable, and which have excellent adhesion with an organic semiconductor material have been expected. An organic transistor having favorable field effect mobility and excellent durability can be obtained by using such a source electrode and a drain electrode.
In addition, a source electrode and a drain electrode also serve as a wiring in an organic field effect transistor; therefore, high conductivity is required. However, a source electrode and a drain electrode with the characteristics as described above and high conductivity have not yet been reported.